Another problem is that Trac tickets are updated if commits contain
references to pull requests. For example, merging PR #382 into master
updated ticket #382 among others.
I appreciate that someone has probably worked on the problem recently
(I notice that PR references no longer lead to comments in Trac tickets)
but there are still annoying e-mail notifications in my mailbox that
suggest that something remains switched on that should be switched off.

Will you be moving over all the documentation from trac/wiki to GH?
There appears to be a lot of outdated documentation scattered throughout the site.

It's because the trac is broken.
The registration is down and so is the bug reporting. That's why they moved to github.
Also note it could take a long time to bring all of this to the github issues.
Like i said.